Violet Red Bile Research Articles

Productivity of Boiled and Autoclaved Violet Red Bile Agar

Violet Red Bile (VRB) Agar which was boiled had significantly greater productivity than did autoclaved VRB Agar when a 4-h Escherichia coli culture was enumerated. There was no significant difference between the boiled and autoclaved medium when a 16-h culture was examined.

Repair and enumeration of injured coliforms by a plating procedure.

Surface plating of coliforms on Trypticase soy agar, followed by 1 to 2 h of incubation at 25 C and subsequent overlay with violet red bile agar, was found to be a useful method for the repair and enumeration of coliforms injured by freezing.

Longevity of Dehydrated Violet Red Bile Agar

The length of storage in sealed containers at room temperature did not affect the plating efficiency of violet red bile agar.

Violet Red Bile 2 Agar for Stressed Coliforms

Violet Red Bile 2 Agar for Stressed Coliforms

Longevity of Dehydrated Violet Red Bile Agar1

The length of storage in sealed containers at room temperature did not affect the plating efficiency of violet red bile agar.

Violet Red Bile 2 Agar for Stressed Coliforms1

Counts on a new, autoclave-sterilizable violet red bile (VRB-2) agar were compared with counts on freshly boiled VRB agar. Yields on VRB-2 agar averaged 217, 180, 130, and 112% of counts obtained on the control medium for samples of water, cottage cheese, frozen vegetables, and raw milk, respectively. The general principle used for the development of VRB-2 agar could be applied to many other kinds of selective plating media.

Repair and Enumeration of Injured Coliforms by a Plating Procedure

Surface plating of coliforms on Trypticase soy agar, followed by 1 to 2 h of incubation at 25 C and subsequent overlay with violet red bile agar, was found to be a useful method for the repair and enumeration of coliforms injured by freezing.

CHARACTERISTICS OF CHEDDAR CHEESE COOLED AT DIFFERENT RATES DURING EARLY CURING STAGES

Sixteen lots of Cheddar cheese were made from manufacturing-grade and grade-A milk by using commercial starters and cultures reported to give fruity and bitter-flavored cheese. Curd blocks were pressed for 4 and 20 h, then cooled to 7.5 C in brine or in air. There were no statistically significant differences in total, enterococcus, and Violet red bile agar counts, or proteolysis and judging scores among different pressing or cooling treatments. Significantly greater amounts of lactose, glucose, and galactose were present in brine-cooled cheese. Lactose persisted beyond 3 months. Air-cooled cheese had a significantly higher lactic-acid content, more free fatty acids, and more color variation. Air-cooled cheese was more severely criticized for flavor defects. Brine cooling produced uniform flavor, body, and color, and six of the seven judges preferred brine-cooled cheese.

Repair and enumeration of injured coliforms in frozen foods.

Two strains of Escherichia coli manifested death and repairable injury after being frozen in water or sterile foods at -20 C. The injured survivors were inhibited from forming colonies on violet red bile agar (VRBA) or deoxycholate lactose agar; this inhibition was greater when enumeration was done by the pour plate method instead of the surface or surface-overlay method. Injured cells repaired rapidly in Trypticase soy broth (TSB), and the repair was about maximum after 1 h at 25 C. When the injured cells were added to different foods and incubated at 25 C, repair also occurred; however, recovery was better and more uniform when the samples were mixed with TSB and incubated 1 h at 25 C. Cell multiplication was not evident until after 90 to 120 min at 25 C. The enumeration of coliforms from commercially frozen foods was increased when the thawed samples were mixed with TSB and the cells were allowed to repair 1 h at 25 C. In some samples, the repair permitted at least a 20-fold increase in the coliform count. The associated flora in the commercially frozen foods gave no evidence of impairing the repair of coliforms, nor did they start multiplication prior to 90 min after being incubated in TSB at 25 C. Generally, the plating gave more reproducible recovery of coliforms than did the most probable number method. Also, a higher number of coliforms were obtained by the surface-overlay method of plating using VRBA.

Heat Injury and Resuscitation of Escherichia coli

Sublethal heat treatment of two strains of Escherichia coli caused injury of the cells to be expressed, when they were plated on violet red bile agar compared to nutrient agar. Subsequent incubation in tryptic soy broth at either 20 or 37°C supported the resuscitation of the cells from injury, before the end of the lag phase. However, at a typical refrigeration temperature (4.4°C) resuscitation did not occur. In addition to resuscitation for growth on violet red bile agar, resuscitation for growth on the nonselective control medium (nutrient agar) also occurred.

Reliability of Selective Media for the Enumeration and Estimation of Escherichia coli

Many solid and liquid media are available for the isolation and identification of coliform bacteria. Since standards exist for coliform bacteria in some food products, reliable methods for enumerating these organisms, especially after exposure to conditions causing injury, are necessary. Solid media, such as violet red bile agar, desoxycholate agar, desoxycholate lactose agar and MacConkey agar all caused a marked underestimation of the viable counts of heat injured E. coli. Eosin methylene blue agar was eliminated from this work as a medium for enumerating coliform bacteria because its dark colour made counting of the colonies difficult. The liquid media also underestimated the viable counts of heat injured E. coli. BDG (Buffered desoxycholate glucose) broth gave the best results for the estimation of the injured cells, however more trials on a wider range of coliform bacteria would be required before this medium could be recommended for the estimation of coliform densities. Generally, it may be concluded that caution should be exercised in the interpretation of coliform estimates from samples which might contain stressed cells.

Effect of Diamide on Growth of selected Bacteria

Selected species of Pseudomonas, Alcaligenes, Enterobacter, Escherichia, Streptococcus, and Lactobacillus were tested for growth in trypticase soy broth, trypticase soy agar, plate count agar, and violet red bile agar containing .025 or .05% diamide (diazenedicarboxylic acid bisdimethylamide). None of the resulting media was suitable for selectively determining species of Pseudomonas and Alcaligenes, but adding .05% diamide to trypticase soy agar or violet red bile agar apparently made both suitable for determining Pseudomonas fluorescens.

CONDITION OF COLIFORM ORGANISMS INFLUENCING RECOVERY OF SUBCULTURES ON SELECTIVE MEDIA

The effects of stress of coliform bacteria resulting from exposure to heat, radiation, or sodium chloride on behavior of the progeny were studied. After exposure to stress and subsequent growth on plating media, colonies were picked by random design and grown in nutrient broth for further comparison of their ability to form colonies on plate count and violet red bile agar. After 6 hr in nutrient broth, average counts on violet red bile agar were less than half those obtained with plate count agar. Sensitivity to the selective medium was lost by repeated transfer and growth in nutrient broth or by repeated picking from the selective medium and subculture of colonies.Cultures with moderate sensitivity to violet red bile agar were obtained from raw sewage through picking of colonies from plate count agar. Attempts to obtain a stable sensitive strain through selective enrichment were unsuccessful. A laboratory strain of Escherichia coli, with extreme sensitivity to violet red bile agar, however, was used to determine that tolerance was acquired by stepwise adaptation to a selective medium. Occurrence of sensitive coliform cells in nature indicates their potential importance in tests for indicator organisms of public health significance.

Repair and Enumeration of Injured Coliforms in Frozen Foods1

Two strains of Escherichia coli manifested death and repairable injury after being frozen in water or sterile foods at -20 C. The injured survivors were inhibited from forming colonies on violet red bile agar (VRBA) or deoxycholate lactose agar; this inhibition was greater when enumeration was done by the pour plate method instead of the surface or surface-overlay method. Injured cells repaired rapidly in Trypticase soy broth (TSB), and the repair was about maximum after 1 h at 25 C. When the injured cells were added to different foods and incubated at 25 C, repair also occurred; however, recovery was better and more uniform when the samples were mixed with TSB and incubated 1 h at 25 C. Cell multiplication was not evident until after 90 to 120 min at 25 C. The enumeration of coliforms from commercially frozen foods was increased when the thawed samples were mixed with TSB and the cells were allowed to repair 1 h at 25 C. In some samples, the repair permitted at least a 20-fold increase in the coliform count. The associated flora in the commercially frozen foods gave no evidence of impairing the repair of coliforms, nor did they start multiplication prior to 90 min after being incubated in TSB at 25 C. Generally, the plating gave more reproducible recovery of coliforms than did the most probable number method. Also, a higher number of coliforms were obtained by the surface-overlay method of plating using VRBA.

EVALUATION OF METHODS FOR DETECTION OF COLIFORM BACTERIA IN RAW MILK

Estimates of populations of “total” coliform bacteria in raw mi1k were quantitatively essentially the same when Violet Red Bile agar incubated at 32 C and Lauryl Sulfate Tryptose Broth incubated at 37 C were used. Qualitatively, LST tended to favor recovery of a greater proportion of Aerobaoter aerogenes and IMViC-irregular types. Of the 46 isolates that produced ropiness, 35 were recovered from LST.Escherichia coli populations, as determined on EC medium incubated at 44.5 C, showed no constant relationship to total coliform population. Summer samples contained not only greater numbers of E. coli but also these organisms constituted a greater proportion of the coliform population.

Heat Survivor Curves of Food-Borne Bacteria Suspended in Commercially Sterilized Whole Milk. II. Bacteria Other than Salmonellae

Heat survivor curves at 55, 60, and 62.5C of a number of food-borne bacteria of significance to the milk industry, suspended in commercially sterilized whole milk, were generally nonlogarithmic. The inability of heated Escherichia coli to grow on violet red bile agar and of heated Staphylococcus aureus to grow on staphylococcus agar 110 was confirmed. Enterococci counts as indices of post-heat contamination were not applicable. The effect of recovery test on maximum survival times and the need for recovery test in the enumeration and detection of bacteria in heat treated milk were emphasized.

Acid injury of Escherichia coli.

Two strains of Escherichia coli sublethally treated in acidic environments showed an increased sensitivity to violet red bile agar. The death of cells, treated in tryptic soy broth which had been acidified with two inorganic and five organic acids, was followed on both nutrient agar and the selective medium violet red bile agar. The death rates determined on violet red bile agar were much faster than those established on nutrient agar. Therefore, it appeared that unreliable results may occur if selective media are used for the detection and enumeration of Enterobacteriaceae in acid foods.

Inhibition of Injured Escherichia coli by Several Selective Agents

A population of Escherichia coli ML30 cells was exposed to a quaternary ammonium compound, and injury to the cells was measured by a comparison of counts on Trypticase Soy Agar and Violet Red Bile Agar. Substantial injury could not be detected with a minimal medium. The ingredients of Violet Red Bile Agar were tested against damaged cells. The bile salts mixture alone in the medium prevented as many injured cells from growing as did any combination of the selective agents and inhibited as many injured bacteria as were inhibited by Violet Red Bile Agar itself. These dyes and salts were similarly assayed in minimal agar, and comparable results were obtained. Individual bile salts and other potential selective agents were added to the minimal medium, and the media were tested for inhibition of injured E. coli. Sodium deoxycholate was the bile salt most inhibitory to damaged E. coli cells.

Examination of Coliforms in Frozen Dairy Products

Considering the cellular injury of microorganisms caused by the processing operation, the detection of coliforms in frozen dairy products was examined in a model system. Viable counts of test organisms (Escherichia-Aerobacter group) frozen and stored at -20°C in sterilized skim milk were determined by selective and nonselective media. Ten to 73% of viable population was unable to reproduce on desoxycholate agar and violet red bile agar. However, the frozen injured cells displayed no sensitivity to brilliant green lactose bile broth. The percentage of injured survivors became greater when the cells were frozen and maintained in skim milk of relatively low concentration of solids-not-fat. Sucrose exerted a considerable protective action against the injury due to freezing. The present results suggest that the sublethal injury should be considered carefully in the microbiological evaluation of frozen dairy products.

Inhibition of Injured Escherichia coli by Several Selective Agents 1

A population of Escherichia coli ML30 cells was exposed to a quaternary ammonium compound, and injury to the cells was measured by a comparison of counts on Trypticase Soy Agar and Violet Red Bile Agar. Substantial injury could not be detected with a minimal medium. The ingredients of Violet Red Bile Agar were tested against damaged cells. The bile salts mixture alone in the medium prevented as many injured cells from growing as did any combination of the selective agents and inhibited as many injured bacteria as were inhibited by Violet Red Bile Agar itself. These dyes and salts were similarly assayed in minimal agar, and comparable results were obtained. Individual bile salts and other potential selective agents were added to the minimal medium, and the media were tested for inhibition of injured E. coli. Sodium deoxycholate was the bile salt most inhibitory to damaged E. coli cells.